In some athletes with a substantial increase in left ventricular wall thickness, it may be difficult to distinguish with certainty physiologic hypertrophy due to athletic training from hypertrophic cardiomyopathy with relatively mild wall thickening. To assess the potential role of Doppler echocardiographic indexes of diastolic filling in differentiating these two conditions, we measured Doppler diastolic indexes in 16 young competitive athletes with increased left ventricular wall thickness (range 13-16 mm) and in 12 young asymptomatic patients with nonobstructive hypertrophic cardiomyopathy and similar magnitude of left ventricular hypertrophy (range 13-18 mm). In the athlete group, the mean values for each measured diastolic index was not significantly different from the mean values obtained in untrained normal subjects. Conversely, in the patients with hypertrophic cardiomyopathy, mean values for each of the four Doppler diastolic indexes were significantly different from normal (p<0.02), and one or more indexes were abnormal in 10 (83%) of the 12 patients. These findings indicate that Doppler echocardiographic indexes of left ventricular filling may provide additional useful information and aid in distinguishing marked physiologic hypertrophy due to athletic training from pathologic hypertrophy associated with nonobstructive hypertrophic cardiomyopathy.